In a lead-acid battery, a paste type electrode plate filled with paste mainly composed of lead powder or lead oxides such as litharge (PbO) is mostly used in the grid made of lead (Pb) or Pb alloy. As the grid, hitherto, a casting grid has been widely used. Recently, however, a paste type electrode plate using an expanded grid is being widely used because of its excellent mass producibility. The paste type electrode plate using the expanded grid is suited to a thin electrode plate, and it is appropriate for composing a battery for an automobile or an electric vehicle in which heavy load discharge characteristics are typically required.
As the Pb alloy for the grid, a lead-antimony (Pb--Sb) system and a lead-calcium (Pb--Ca) system are representative alloys.
The Pb--Sb system alloy is smooth in flow of molten alloy in casting, and excellent in casting performance, and is also high in mechanical strength of the grid. Hence it has been a common casting grid. However, when the Pb--Sb system alloy high in Sb content is used in the positive electrode grid, Sb dissolving from the alloy into the electrolyte deposits on the negative electrode surface. Since Sb is low in hydrogen overvoltage and is likely to generate hydrogen, it causes a decrease in the electrolyte amount when charging or overcharging. Recently, accordingly, there is a tendency of using the so-called low antimony lead alloy having the Sb content in the Pb--Sb system alloy decreased to about 2.5 wt. % of the allowable range of mechanical strength (about half of the conventional content), in the positive electrode plate.
By contrast, in the battery using the grid made of Pb--Ca system alloy, the advantage is the small extent of decrease of electrolyte amount by charging and discharging cycle and self-discharge in charged state. Casting performance, however, has been poor, the crystal particle size of alloy is larger than in the Pb--Sb system alloy and corrosion resistance is lowered, and the mechanical strength of the grid after casting is low. Accordingly, the casting grid by Pb--Ca system alloy has been limited to small-size batteries only.
On the other hand, as the Pb--Ca system alloy for expanded grid, hitherto, the Pb--Ca--Sn alloy adding tin (Sn) by about 0.25 wt. % has been generally used for positive and negative electrode plates. The battery using this alloy grid, when charged at constant voltage following a long-term standing after discharge, reaches the setting voltage in an extremely short time, and is substantially sometimes unable to charge. The cause of such phenomenon is known to exist in the positive electrode. That is, the sulfuric acid in the electrolyte is consumed by discharge, and the electrolyte near the positive active material becomes neutral. When stored for a long-term in this state, a passive layer expressed by PbOx (x=1 to 1.5) is formed in the interface between the grid and the active material in the positive electrode, and the internal resistance of the battery increases, which is considered to cause this phenomenon.
Also in the battery using the expanded grid made of Pb--Ca system alloy in the positive electrode plate, when charging and discharging by constant voltage charging are repeated at a high temperature of 40.degree. C. or more, in particular, over 70.degree. C., the positive electrode is changed in shape due to stretch by corrosion of the grid, and decreased in capacity due to softening or drop-out by lowering of binding force between active material particles. In the negative electrode, too, decrease of capacity due to shrinkage by increase ot particle size ot active material is noted. Such phenomenon is observed not only in the Pb--Ca system alloy, but also in the grid using pure Pb.
To suppress formation of the passive layer between the grid of pure Pb or Pb--Ca system alloy and the active material in the positive electrode, it is known that increase of the content of Sn or Sb in the pure Pb or Pb--Ca system alloy is effective. However, when the content of Sn or Sb is increased, the alloy price is raised, and the battery manufacturing cost is increased. Moreover, when the total Sn content is increased, an internal short circuit is caused by dissolution and deposition of Sn, which causes a decrease of the cycle life. And, by increasing the total content of Sb, as mentioned above, Sb is dissolved and Sb precipitates on the negative electrode, and the electrolyte amount is decreased due to charging and discharging cycles, and self-discharge increases.
It was hence attempted to suppress the formation ot the passive layer by raising the Sn and/or Sn content in part of the grid surface contacting with the active material in the positive electrode. More specifically, a sheet of Pb--Sn alloy is overlaid on a Pb--Ca system alloy parent material, and integrated by cold rolling process to obtain a clad sheet, from which a grid is manufactured by expanded processing or perforated processing, as disclosed in Japanese Patent Publication No. Hei04-81307, and a Pb--Sn alloy or Pb--Sn--Sb alloy layer with Sb concentration of 0.3 wt. % or less is formed on a Pb--Ca system alloy parent material, and then a Pb--Sb alloy or a Pb--Sn--Sb alloy layer with Sb concentration of 0.8 wt. % or more is integrally formed thereon to obtain a clad sheet, from which a grid is manufactured by expanded processing or perforated processing, as disclosed in Japanese Laid-open Patent Application No. Sho61-200670.
By using these grids in the positive electrode plate, while maintaining the advantages of the conventional Pb--Ca system alloy grid in which decrease of electrolyte amount due to charging and discharging cycles and self-discharge are suppressed, it was expected that these grids would be an effective means for solving a number of problems. The problems expected to be solved relate to charging reception characteristic after long-term storage following deep discharge, shape change of grid in the positive electrode, and decrease of capacity of positive and negative electrodes by charging and discharging cycles at high temperature.
However, these effects were not sufficiently addressed by these grids, and the degree of which the effects were addressed may vary.